1. Field of the Invention
The present invention relates to an electric control apparatus for a transfer device in part-time four-wheel drive vehicles, the transfer device being arranged to be operated by the driver to alternately provide a two-wheel drive mode or four-wheel drive mode of operation for the vehicle.
2. Discussion of the Background
In Japanese Patent Early Publication No. 59-216732, there has been proposed a transfer device of this kind which includes an input shaft driven by a power transmission and two output shafts, one of the output shafts being drivingly connected to the input shaft for continuously driving one set of rear road wheels through a rear propeller shaft, a rear differential and split axle parts, and the other output shaft being connectable to the input shaft by a clutch mechanism in the transfer device for selectively driving a set of front road wheels through a front propeller shaft, a front differential and split axle parts. In the transfer device, a two-wheel drive mode is provided by disengagement of the clutch mechanism, and a four-wheel drive mode is provided by engagement of the clutch mechanism. In such a part-time four-wheel drive system, conventional hub locks are used in connection with the front road wheels to disconnect the wheels from their associated split axle parts when the two-wheel drive mode is selected at the transfer device. This is effective to eliminate back drive of the split axle parts, differential and propeller shaft for the non-driven front road wheels.
Assuming that the vehicle is travelling at a mid or high speed under unlocked condition of the hub locks in the two-wheel drive mode, the split axle parts, differential and propeller shaft for the non-driven front road wheels are still, while the split axle parts, differential and propeller shaft for the rear road wheels are driven at a mid or high speed. If the clutch in the transfer device is engaged to select the four-wheel drive mode during such mid or high speed travel of the vehicle, drive connection between the input shaft and the non-driven output shaft for the front propeller shaft will cause a great impact acting on the transfer device. This causes unpleasant shocks, noises and unexpected damage of component parts in the transfer device. To solve such problems, the driver is obliged to select the four-wheel drive mode at the transfer device only when the vehicle is stopping. This results in inconvenience for the vehicle driver and deteriorates the merit of the part-time four-wheel drive system.